Homolytic Chlorination of Aliphatic Compounds. I. Photochlorination of Chloroethanes

The effect of the structure on the rate of hydrogen abstraction from chloroalkanes by a chlorine atom has been investigated by carrying out the competitive photochlorination of eight different chloroethanes in the presence of 2,2-dichloropropane to which the relative site reactivities have been referred. The results show that the successive introduction of a chlorine sub-stituent at the 1-position decreases the reactivity of the 2-position, but that, in some cases, it increases that of the 1-position. A linear free-energy relationship is found to exist between the reactivities of the R group in R-R′ (R=methyl, chloromethyl or dichloromethyl; R′=methyl, chloromethyl, dichloromethyl or trichloromethyl) and Taft’s σ* constants for the R′ group. These facts are elucidated in terms of the inductive and resonance effects of the substituent groups.